Carbon is known to have four unique crystalline structures, including diamond, graphite, fullerene and carbon nano-tubes. The carbon nano-tube (CNT) refers to a tubular structure grown with a single wall or multi-wall, which can be conceptually obtained by rolling up a graphene sheet or several graphene sheets to form a concentric hollow structure. A graphene sheet is composed of carbon atoms occupying a two-dimensional hexagonal lattice. Carbon nano-tubes have a diameter on the order of a few nanometers to a few hundred nanometers. Carbon nano-tubes can function as either a conductor or a semiconductor, depending on the rolled shape and the diameter of the tubes. Its longitudinal, hollow structure imparts unique mechanical, electrical and chemical properties to the material. Carbon nano-tubes are believed to have great potential for use in field emission devices, hydrogen fuel storage, rechargeable battery electrodes, and composite reinforcements.
However, CNTs are extremely expensive due to the low yield and low production and purification rates commonly associated with all of the current CNT preparation processes. The high material costs have significantly hindered the widespread application of CNTs. Rather than trying to discover much lower-cost processes for nano-tubes, we have worked diligently to develop alternative nano-scaled carbon materials that exhibit comparable properties, but can be produced in larger quantities and at much lower costs. This development work has led to the discovery of processes for producing individual nano-scaled graphite planes (individual graphene sheets) and stacks of multiple nano-scaled graphene sheets, which are collectively called “nano-scaled graphene plates (NGPs).” NGPs could provide unique opportunities for solid state scientists to study the structures and properties of nano carbon materials. The structures of these materials may be best visualized by making a longitudinal scission on the single-wall or multi-wall of a nano-tube along its tube axis direction and then flattening up the resulting sheet or plate. Studies on the structure-property relationship in isolated NGPs could provide insight into the properties of a fullerene structure or nano-tube. Furthermore, these nano materials could potentially become cost-effective substitutes for carbon nano-tubes or other types of nano-rods for various scientific and engineering applications.
Direct synthesis of the NGP material had not been possible, although the material had been conceptually conceived and theoretically predicted to be capable of exhibiting many novel and useful properties. Jang and Huang have provided an indirect synthesis approach for preparing NGPs and related materials [B. Z. Jang and W. C. Huang, “Nano-scaled Graphene Plates,” U.S. Pat. No. 7,071,258 (Jul. 4, 2006)]. Another process developed by B. Z. Jang, et al. [“Process for Producing Nano-scaled Graphene Plates,” U.S. patent pending, Ser. No. 10/858,814 (Jun. 3, 2004)] involves (1) providing a graphite powder containing fine graphite particles (particulates, short fiber segments, carbon whisker, graphitic nano-fibers, or combinations thereof) preferably with at least one dimension smaller than 1 μm); (2) exfoliating the graphite crystallites in these particles in such a manner that at least two graphene planes are either partially or fully separated from each other, and (3) mechanical attrition (e.g., ball milling) of the exfoliated particles to become nano-scaled to obtain NGPs. The starting powder type and size, exfoliation conditions (e.g., intercalation chemical type and concentration, temperature cycles, and the mechanical attrition conditions (e.g., ball milling time and intensity) can be varied to generate, by design, various NGP materials with a wide range of graphene plate thickness, width and length values. Ball milling is known to be an effective process for mass-producing ultra-fine powder particles. The processing ease and the wide property ranges that can be achieved with NGP materials make them promising candidates for many important industrial applications. The electronic, thermal and mechanical properties of NGP materials are expected to be comparable to those of carbon nano-tubes; but NGP will be available at much lower costs and in larger quantities.
In this and other methods for making separated graphene or other non-carbon inorganic platelets, the process begins with intercalating lamellar flake particles with an expandable intercalation agent (intercalant), followed by expanding the intercalant to exfoliate the flake particles. Conventional intercalation methods and recent attempts to produce exfoliated products or separated platelets are given in the following representative references:    1. J. W. Kraus, et al., “Preparation of Vermiculite Paper,” U.S. Pat. No. 3,434,917 (Mar. 25, 1969).    2. L. C. Olsen, et al., “Process for Expanding Pyrolytic Graphite,” U.S. Pat. No. 3,885,007 (May 20, 1975).    3. A. Hirschvogel, et al., “Method for the Production of Graphite-Hydrogensulfate,” U.S. Pat. No. 4,091,083 (May 23, 1978).    4. T. Kondo, et al., “Process for Producing Flexible Graphite Product,” U.S. Pat. No. 4,244,934 (Jan. 13, 1981).    5. R. A. Greinke, et al., “Intercalation of Graphite,” U.S. Pat. No. 4,895,713 (Jan. 23, 1990).    6. F. Kang, “Method of Manufacturing Flexible Graphite,” U.S. Pat. No. 5,503,717 (Apr. 2, 1996).    7. F. Kang, “Formic Acid-Graphite Intercalation Compound,” U.S. Pat. No. 5,698,088 (Dec. 16, 1997).    8. P. L. Zaleski, et al. “Method for Expanding Lamellar Forms of Graphite and Resultant Product,” U.S. Pat. No. 6,287,694 (Sep. 11, 2001).    9. J. J. Mack, et al., “Chemical Manufacture of Nanostructured Materials,” U.S. Pat. No. 6,872,330 (Mar. 29, 2005).    10. Morrison, et al., “Forms of Transition Metal Dichalcogenides,” U.S. Pat. No. 4,822,590 (Apr. 18, 1989).
One common feature of these methods is the utilization of liquid or solution-based chemicals to intercalate graphite or other inorganic flake particles. These chemicals often comprise strong acids (e.g., sulfuric and nitric acids), solvents, or other undesirable species that can reside in the material. For instance, the most commonly used method of graphite intercalation for the production of expandable graphite in industry involves immersing graphite in a mixture of sulfuric and nitric acids, to which is added some sodium perchlorate, followed by HCl treatments and a lengthy water rinsing steps. In another example, Mack, et al. [Ref. 9] intercalated laminar materials with alkali metals (e.g. Li, Na, K, Rb, and Cs), alkaline earth metals (e.g. Mg, Ca, Sr, and Ba), Eu, Yb, or Ti. Intercalation of these elements was accomplished by five different routes: (1) intercalated electrochemically using a non-aqueous solvent; (2) using an alkali plus naphthalene or benzophenone along with a non-aqueous solvent (usually an ether such as tetrahydrofuran); (3) using amalgams (metal+mercury); (4) dissolving any of the afore-mentioned metals in a liquid ammonia solution to create solvated ions; and (5) using n-butyl lithium in a hydrocarbon solvent (e.g., hexane).
In addition to the utilization of undesirable chemicals, in most of these methods of graphite intercalation and exfoliation, a tedious washing step is required, which produces contaminated waste water that requires costly disposal steps. Furthermore, conventional exfoliation methods normally involve a very high furnace temperature (typically between 500° C. and 2,500° C.) since the process depends on the decomposition of an intercalant (e.g., NO3−1 and SO4−2) to form expandable gaseous species (e.g., SO2 and NOx). In another approach, intercalation with an alkali or alkaline earth metal entails immersing the layered material in a metal compound solution (rather than pure metal), allowing the metal ions to penetrate into the inter-layer galley (interstitial space). Typically, metal ion content is relatively low compared to other elements in such a compound solution (e.g., in a solution of 20% by weight lithium chloride in water, lithium content is only 3.27% by weight). Hence, only a small amount of ions from a relatively dilute solution penetrates and stays sporadically in these spaces. The resulting exfoliated product often exhibits platelets of widely varying thicknesses and many incompletely delaminated layers. Direct intercalation of graphite with a pure alkali metal is possible [Ref. 9], but has been limited to a laboratory operation (inside a dry glove box, for instance filled with an inert gas) due to the fact that alkali metals such as Li, Na, and K are extremely sensitive to even a trace amount of water and react very violently with water. Such a process is not amenable to mass production of nano-scaled platelets.
In one of our earlier inventions [B. Z. Jang, A. Zhamu, and J. Guo, “Process for Producing Nano-scaled Platelets and Nanocomposites,” U.S. patent application Ser. No. 11/509,424 (Aug. 25, 2006)], a process was provided for exfoliating a layered (laminar) material to produce nano-scaled platelets. The process comprises: (a) subjecting a layered material to a gaseous environment at a first temperature and first pressure sufficient to cause gas species to penetrate (into the interstitial space, also referred to as the interlayer galley) between layers of the layered material, forming a gas-intercalated layered material; and (b) subjecting the gas-intercalated layered material to a second pressure, or a second pressure and a second temperature, allowing gas species to pressurize and expand the interstitial space between layers (and to partially or completely escape from the layered material), thereby exfoliating the layered material to produce the platelets. A high first pressure was used to facilitate penetration of gas species into the interlayer galley to form a tentative gas-intercalated compound. A typically much higher second temperature (than the first temperature) was used to pressurize the interlayer galley. A very short period of time (typically seconds) elapsed between step (a) (once the gas pressure is relieved) and step (b). The gas molecules or atoms reside in the interlayer galley only on a temporary basis and can easily leave the interlayer galley without causing exfoliation if step (b) is not conducted immediately or soon after intercalation. The gas species used in this earlier study included hydrogen, helium, neon, argon, nitrogen, oxygen, fluorine, carbon dioxide, or a combination thereof. These molecules or atoms have one thing in common: having an extremely low melting point and low boiling point (typically lower than −150° C. with the exception of CO2, which has a boiling point of −78° C. and melting point of −57° C.). At room temperature, they are all highly volatile gases and are kinetically too active to stay in the interlayer galley of a layered material (e.g. graphite) for an extended period of time. Hence, they are not capable of forming a stable graphite intercalation compound (GIC).
By contrast, the present inventors, after an intensive study, have come to discover that selected halogen molecules such as bromine (Br2), iodine (I2), iodine chloride (ICl), iodine bromide (IBr), bromine chloride (BrCl if maintained at T<5° C.), iodine pentafluoride (IF5), bromine trifluoride (BrF3), chlorine trifluoride (ClF3, if maintained at T<11.8° C.), or a combination thereof can be used to intercalate a range of layered materials to form stable intercalation compounds at room temperature. The resulting intercalation compounds could remain stable for a long time (e.g., bromine-intercalated natural graphite remains stable for many months), enabling the compound to be exfoliated when and where the platelets are needed. The halogen molecules or compounds in a vaporous state are capable of forming low-stage intercalated compounds (e.g., stage 1, 2, 3, 4 and 5). For the intercalated graphite, stage n implies that there are n graphene sheets between two intercalant layers (typically each intercalant layer inside an interlayer galley is a monolayer of intercalant molecules or atoms). After exfoliation of intercalated graphite particles of stage 1, 2, and 3, for instance, the resulting platelets tend to be single-sheet, double-sheet, and triple sheet graphene platelets. The presently invented method has an excellent control over the nano platelet thickness. Furthermore, most of the halogen molecules can be recovered and re-used. We have further found that the resulting nano-scaled platelets can be readily dispersed in a range of liquid media, making it possible to prepare nanocomposites. The same method is also found to be suitable for the production of other nano-scaled inorganic platelets, for instance, dichalcogenides, such as MoS2, which have found applications as electrodes in lithium ion batteries and as hydro-desulfurization catalysts.
It may be noted that Martin, et al. [W. H. Martin and J. E. Brocklehurst, “The Thermal Expansion Behavior of Pyrolitic Graphite-Bromine Residue Compounds,” Carbon, 1 (1964) 133-141] observed the “breakaway” expansion of pyrolytic graphite-bromine residue compound. For unknown reasons, their bromine-intercalated pyrolytic graphite compounds appear to be unstable with a majority of bromine escaping from graphite upon completion of intercalation, leaving behind at most a Br/C atomic ratio of 4%. Hence, only a limited amount of bromine remained in a limited number of interlayer spaces of a graphite sample. This amount and number were sufficient to induce delamination of only a limited number of layers (there is a large number of graphene layers between two delaminations). As a consequence, the irreversible expansion of these graphite-bromine residue compounds resulted in a low-extent, incomplete exfoliation only. The resulting platelets are not fully separated and are not nanometer-scaled.
It is therefore an object of the present invention to provide an environmentally benign method of exfoliating a laminar (layered) compound or element, such as graphite, graphite oxide, and transition metal dichalcogenides, without using undesirable intercalating chemicals.
It is another object of the present invention to provide a convenient method of exfoliating a laminar compound or element to produce nano-scaled platelets (platelets with a thickness smaller than 100 nm, mostly smaller than 10 nm, typically smaller than 1 nm) where and when these platelets are needed.
It is still another object of the present invention to provide a method of producing nano-scaled platelets that can be readily dispersed in a liquid to form a nanocomposite structure.
Another object of the present invention is to provide a relatively low-temperature process for producing nano-scaled platelets with relatively uniform thicknesses.